1. Field of the Invention
This invention relates generally to the field of enclosures for housing electronics, and in particular to a system for dissipating static electric charge from electronics on a circuit board as the board is inserted into the enclosure.
2. Description of the Related Art
Enclosures for housing electronic circuit boards are widely used in a variety of technologies, including telecommunications, industrial control, medical electronics, and military applications. Such enclosures are provided with guides so that individual circuit boards can be slid in or out of the enclosure, and a backplane through which the circuit boards may be electrically connected to other electronic components. Each circuit board is electronically connected to the backplane through engagement of pins projecting from the backplane with connectors attached to the circuit boards.
When a circuit board is inserted in an enclosure, static from the technician may be discharged onto the circuit board. It is important that this static be discharged from the circuit board prior to the connectors of the circuit board making contact with the pins of the backplane. Otherwise, the static will travel through the backplane pins toward electronic components connected to those pins and may damage those components.
In electronics systems, many devices and techniques have been developed to provide electrostatic discharge (ESD) protection and to provide electromagnetic containment (EMC) or electromagnetic interference (EMI) shielding. These devices and techniques include those disclosed in the following references, each of which is incorporated by reference it its entirety herein:                U.S. Pat. No. 3,835,438, issued Sep. 10, 1974 to Longworth, Jr., entitled “Self-Grounding Clip For The Mounting Strap Of An Electric Switch Or Receptacle”;        U.S. Pat. No. 4,781,603, issued Nov. 1, 1988 to Olsson et al., entitled “Grounding System For A Cabinet”;        U.S. Pat. No. 5,196,712, issued Mar. 23, 1993 to Nguyen et al., entitled “Printed Circuit Board Apparatus With Optical Switching”;        U.S. Pat. No. 5,311,408, issued May 10, 1994 to Ferchau et al., entitled “Electronic Assembly With Improved Grounding And EMI Shielding”;        U.S. Pat. No. 5,478,253, issued Dec. 26, 1995 to Biechler et al., entitled “Electrostatic Discharge Contacts For Blind Mating Connectors”;        U.S. Pat. No. 5,537,294, issued Jul. 16, 1996 to Siwinski, entitled “Printed Circuit Card Having A Contact Clip For Grounding A Printed Circuit Board Found Therein”;        U.S. Pat. No. 5,867,366, issued Feb. 2, 1999 to Klein et al., entitled “Electronic Module And Plastic Substrate To Accept And Hold The Electronic Module”;        U.S. Pat. No. 5,916,652, issued Jun. 29, 1999 to Miner et al., entitled “Liner For Adhesive-Backed Sheet Material”;        U.S. Pat. No. 5,934,916, issued Aug. 10, 1999 to Latal et al., entitled “Printed Circuit Board Mounting Rail Member And Ground Clip Assembly”;        U.S. Pat. No. 5,973,926, issued Oct. 26, 1999 to Sacherman et al., entitled “Method And Apparatus For Attaching Circuit Board To Chassis And Forming Solid Ground Connection Using A Single Crew”;        U.S. Pat. No. 6,080,930, issued Jun. 27, 2000 to Lommen et al., entitled “ESD/EMC Gasket”;        U.S. Pat. No. 6,159,024, issued Dec. 12, 2000 to Munch et al., entitled “Device For Creating An Electrically Conducting Connection For An Assembly Panel Fitted In A Switch Cabinet”;        U.S. Pat. No. 6,201,710, issued Mar. 13, 2001 to Bagung et al., entitled “Housing Configuration For A Printed Circuit Board Equipped With Components”;        U.S. Pat. No. 6,254,402, issued Jul. 3, 2001 to Barnes, Jr. et al., entitled “Push Pin Ground”;        U.S. Pat. No. 6,276,947, issued Aug. 21, 2001 to Homfeldt et al., entitled “U-Crimp”;        U.S. Pat. No. 6,282,101, issued Aug. 28, 2001 to Hanas et al., entitled “EMI/ESD Shielding Assembly For An Electronic System”;        U.S. Pat. No. 6,301,124, issued Oct. 9, 2001 to Nikazm et al., entitled “Computer Chassis Identification Method”; and        U.S. Pat. No. 6,483,023, issued Nov. 19, 2002 to Jacques, entitled “Fabric Wrapped Over Spring EMI Gasket”.        
Referring first to FIG. 6, which is a prior art representation of a portion of a printed circuit board 10 being inserted into a conventional enclosure 11. The circuit board 10 may conventionally include ground pins 12 beneath the printed circuit board which fit into receptacles 13 formed through a bottom bracket 16 across the front face of the enclosure. Typically, the bottom receptacles are circular openings sized so that the ground pin 12 at the bottom of the printed circuit board engages the sides of the receptacles 13 prior to the circuit board connectors engaging the backplane pins so that any charge on the circuit boards passes from the pins to the brackets as a result of contact with the sides of the receptacles. The brackets are in turn grounded through the housing of the enclosure.
A problem of the above solution is that the pins must be aligned to the receptacles within a relatively tight tolerance. If the pins do not align properly within the receptacles (for example due to faulty manufacturing or a ground pin being bent), the pins may butt up against the bracket and prevent the circuit board from being inserted into the enclosure. This problem is exacerbated in that typical enclosures may have slots for sixteen or more circuit boards, each having a separate ground pin which must align and be properly received within its respective receptacle. Other conventional enclosures house even more circuit boards.
Even where the pins do fit within the receptacles, there is not always good electrical contact, resulting either in charge being retained on the boards, or charge having to travel across a relatively small contact area between pin and receptacle, thus resulting in high resistances and heat generation.
Referring still to prior art FIG. 6, the enclosure typically includes a second receptacle 15 mounted in a top bracket 14 for receiving an alignment pin 17 on the top portion of the circuit board 10. Owing to the difficulty in aligning both bottom pin 12 and top pin 17 within respective bottom and top receptacles 13 and 15, the top receptacle 15 is typically oblong in shape. (The positions of circular receptacle 13 and oblong receptacle 15 are occasionally switched so that the oblong receptacle 15 is provided in the bottom bracket 16). Due to the oblong shape of receptacle 15, the top pin 17 and receptacle 15 are ineffective as a ground path for the discharge of static electricity from the circuit board 10.